The navigation device to be mounted on an automobile or a two-wheeled vehicle, for example, is constructed to include: position detecting means such as a GPS (Global Positioning System) for determining the current position of the user of the vehicle by using the electric waves coming from a plurality of artificial satellites going around the earth; reproduction means for reproducing storage medium such as the CD-ROM stored with map data and accessory data associated therewith (e.g., display data to be displayed together with the map data such as place names or road names, or non-display data not to be displayed together with the map data, such as coordinates of nodes such as the intersections on roads or the points having made turns at predetermined angles (in degrees) or the distance or the like of an element indicating the segment between the nodes) associated with the map data; and control means such as a microcomputer for controlling those means. On the basis of the current position specified by the position detecting means, the reproduction means reads the map data, as stored in the storage medium, of the corresponding area, and the current position or the like is displayed by display means such as a liquid crystal display element (as referred to JP-A-10-20776 and JP-A-11-14378, for example).
In another navigation device practiced, alternatively, by operating input means such as a remote-control or touch-panel type switch to set a starting place and a final place (including a routed place on the route), the control means searches a scheduled route from the map data or the like of its surrounding place, and the user is guided by using the display means on the basis of the scheduled route and the current position.
This navigation device has to be mounted on the vehicle in its entirety including the position detecting means, the reproduction means (containing the storage medium), the control means, the display means and the input means, so that the system construction is large-sized to raise a problem in the retention of the mounting space.
For the guidance, on the other hand, there is known a navigation device (as referred to JP-A-9-145394), in which the precision is enhanced by using the absolute position obtained by the GPS in longitude and latitude, together with the relative position determined by an self-contained navigation for measuring the movements from a reference point (e.g., the starting point) by accumulating them with a distance sensor and a direction sensor, because it is difficult for the position detecting means to determine the current position precisely merely with the GPS.
This navigation device can have an enhanced location precision but is troubled by a problem that the operations of the absolute position and the relative position are complicated.
There is also known a navigation device, in which the number of branches of routes, the branching directions and the positional information (in latitude, longitude and altitude) are stored as the map information in a flash memory to display those branching situations and the straight lines joining the individual branches in distance and direction, so as to retain the portability and to reduce the capacity of storage for the map information (as referred to JP-A-2002-22480, for example).
In this navigation device, the storage capacity is reduced, but the angular data in the longitude and latitude are used. Therefore, it seems that a difference appears in the distance data which are needed when the relations between the current position and the branches are measured and displayed. This is caused the earth has a spherical shape so that the point distance becomes different even for an equal angular difference.
The present invention has been conceived in view of those problems and has an object to provide a navigation device and a navigation method, which can reduce the size of the system construction and can improve the precision while simplifying the operations.